Recently, a non-contact power transfer (contactless power feeding, also referred to as wireless power feeding) for supplying electronic devices with electric power began to be widely spread. In order to improve interoperability between the products of different manufacturers, the WPC (Wireless Power Consortium) was organized. The WPC has developed a Qi (Chi) standard, which is an international standard.
FIG. 1 is a diagram showing the configuration of a wireless power feeding system 100 according to the Qi standard. The power feeding system 100 is equipped with a power transmitting device 200, such as a TX, a power transmitter, etc., and a power receiving device 300, such as a RX, a power receiver, etc. The power receiving device 300 is mounted in electronic devices such as a cell-phone terminal, a smart phone, an audio player, a gaming device, a tablet terminal, etc.
The power transmitting device 200 has a transmitting antenna 201, a driver 204, a controller 206, and a demodulator 208. The transmitting antenna 201 includes a transmitting coil (e.g., a primary coil) 202 and a resonant capacitor 203. The driver 204 includes an H bridge circuit (e.g., a full-bridge circuit) or a half-bridge circuit. The driver 204 applies a driving signal S1, specifically, a pulse signal, to the transmitting coil 202 and generates a power signal S2 of the electromagnetic field in the transmitting coil 202 by allowing the driving current flow through the transmitting coil 202. The controller 206 is used to control the entire power transmitting device 200. Specifically, the controller 206 changes the transmitting power by controlling the switching frequency of the driver 204 or the duty cycle of switching.
According to the Qi standard, a communications protocol between the power transmitting device 200 and the power receiving device 300 has been defined and it is possible to transmit control data S3 from the power receiving device 300 to the power transmitting device 200. This control data S3 is a type modulated by Amplitude-Modulation (AM) modulated data by using a backscatter modulation and is transmitted from a receiving coil 302, such as a secondary coil, to a transmitting coil 202. For example, this control data S3 may include power control data (also referred to as a packet) indicative of the amount of electric power supplied for the power receiving device 300, data indicative of unique information of the power receiving device 300, etc. The demodulator 208 demodulates control data S3 included in the current or voltage of the transmitting coil 202. The controller 206 controls the driver 204 based on power control data included in the demodulated control data S3.
The power receiving device 300 has a receiving coil 302, a rectifier circuit 304, a capacitor 306, a modulator 308, a secondary battery 310, a controller 312, and a charging circuit 314. The receiving coil 302 may receive a power signal S2 from the transmitting coil 202 and transmit control data S3 to the transmitting coil 202. Both the rectifier circuit 304 and capacitor 306 rectify and smooth a current S4, which is induced in the receiving coil 302 based on the power signal S2, and convert it to a DC voltage.
The charging circuit 314 charges the secondary battery 310 using power supplied from the power transmitting device 200.
The controller 312 monitors the amount of power supply that the power receiving device 300 is receiving, and generates the power control data (e.g., control error value) indicative of the amount of power supply based on the monitored amount of power supply. The modulator 308 modulates the control data S3 containing power control data. In addition, the modulator 308 modulates a coil current and a coil voltage in the transmitting coil 202 by modulating the coil current flowing through the receiving coil 302.
The power feeding system 100 is configured as mentioned above.
FIGS. 2A and 2B show circuit diagrams of the modulator 308 that the present inventors have studied. The receiving antenna 301 includes a receiving coil 302 and a resonant capacitor 303 that are connected in series to each other. The modulator 308 of FIG. 2A includes capacitors C1, C2, switches SW1, SW2 and a resistor R1. When turning on the switches SW1, SW2, a parallel resonance frequency is shifted as compared to that of an off state, thereby obtaining a modulation depth for AM communications. In this manner, the AM modulation can be performed.
The modulator 308 of FIG. 2B includes a switch SW3 and a resistor R3 that are connected in series between the output of the rectifier circuit 304 and the ground. When turning on the switch SW3, a current from the output of the rectifier circuit 304 is flowing to the ground, thereby obtaining the modulation depth for AM communications.
The present inventors have studied the modulator 308 of FIGS. 2A and 2B and have perceived the following problems.
FIG. 3 is a diagram showing the frequency characteristics of the modulator 308 of FIG. 2A. An f1 indicates a series resonance frequency and an f2 indicates a parallel resonance frequency. According to on/off states of the switches SW1, SW2, the parallel resonance frequency f2 may be shifted. In this case, since a modulation depth becomes zero at the cross-point of the two frequency characteristics, this may cause the communication to become unstable if the frequency of the data communications is located in the vicinity of the cross-point.
Additionally, in the case of using the modulator 308 of FIG. 2B, since a portion of the received power is flowing to ground after passing through the switch SW3 and the resistor R3, this may cause a decrease in efficiency and/or heat generation.